E. coli. Coloured transmission electron micrograph (TEM) of an Escherichia coli bacterium. E. coli are Gram-negative bacilli (rod-shaped) bacteria. Long flagellae (light blue) enable the bacterium to be motile; the red filaments on the sides of the bacterium are pili, thin strands of protein used when two bacteria conjugate (transfer DNA). This bacterium is a normal inhabitant of the human intestine. However, under certain conditions its numbers may increase, causing infection. It can cause traveller's diarrhoea" and urinary tract infections such as cystitis. Magnification unknown."